Method of manufacturing membrane electrode assembly, and membrane electrode assembly

ABSTRACT

There is provided a method of manufacturing a membrane electrode assembly that has an electrode catalyst layer formed on a surface of an electrolyte membrane. The electrode catalyst layer formed in the membrane electrode assembly is produced by a drying process that dries a catalyst ink which includes catalyst-supported particles having a catalyst metal supported thereon, a solvent and an ionomer, at a predetermined temperature. The catalyst ink includes a plurality of different solvents having different boiling points. The predetermined temperature is set to be lower than the boiling point of the solvent having the lowest boiling point among the plurality of different solvents.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo. 2014-226629 filed on Nov. 7, 2014, the entirety of disclosure ofwhich is hereby incorporated by reference into this application.

BACKGROUND Field

The present invention relates to a method of manufacturing a membraneelectrode assembly used for a fuel cell, and a membrane electrodeassembly.

Related Art

A membrane electrode assembly (MEA) used for a fuel cell is a powergeneration element including an electrolyte membrane and electrodes(anode and cathode) formed on respective surfaces of the electrolytemembrane. Each of the electrodes includes an electrode catalyst layerthat is placed to be in contact with the electrolyte membrane and a gasdiffusion layer formed on the electrode catalyst layer.

JP 2005-235556A describes a method of manufacturing an electrodecatalyst layer. The method coats a conductive porous body (for example,carbon paper) with a mixture of catalyst particles (for example,platinum-supported carbon fine particles), a hydrogen ion-conductiveresin (for example, Nafion (registered trademark)) and a solvent (forexample, alcohol) (this mixture is called “catalyst ink”) and evaporatesthe solvent in an atmosphere at temperature of 60 to 80° C. to formcatalyst layers. The method subsequently heats the formed catalystlayers at temperature of 100 to 140° C. in an atmosphere that includesoxygen of not lower than 2×10 Pa and not higher than the oxygen partialpressure of the air, so as to produce heat-treated electrode catalystlayers (more specifically, oxygen electrode catalyst layer and fuelelectrode catalyst layer).

JP 2012-212661A describes another method of manufacturing an electrodecatalyst layer. The method disperses catalyst particles, a polymerelectrolyte and first carbon particles in a solvent to prepare a firstcatalyst ink and dries the first catalyst ink at temperature between 30°C. and 140° C. inclusive to obtain a catalyst aggregate. The methodsubsequently disperses the catalyst aggregate and second carbonparticles in a second solvent to obtain a second catalyst ink. Themethod coats a base material with the second catalyst ink and driescoated layers of the second catalyst ink to form electrode catalystlayers.

A fluororesin (for example, Nafion (registered trademark)) that is ahigh-molecular polymer having a sulfonic acid group (—SO₃H) as an endgroup is often used as an electrolyte material or more specifically anionomer included in an electrode catalyst layer. The high-molecularpolymer is likely to be deteriorated (decomposed) from its end group.Radial decomposition by the chemical action during power generation, asthe measure factor, may cause decomposition of the sulfonic acid groupof the ionomer in the electrode catalyst layer and thereby increasesulfate ion (SO₄ ²⁻). This may decrease pH in the fuel cell or morespecifically in the membrane electrode assembly of the fuel cell toprovide an acidic environment and cause poisoning of the electrodecatalyst layer. Poisoning of the electrode catalyst layer may result inreducing the proton conductivity of the electrode catalyst layer andincreasing the impedance of an electrode comprised of the electrodecatalyst layer and a gas diffusion layer and may thus lead to reducingthe power generation performance of the fuel cell. A radical scavenger(for example, cerium oxide) included in the gas diffusion layer of themembrane electrode assembly is used to suppress an increase of sulfateion during power generation.

The inventors of the present application have found that the sulfonicacid group of the ionomer is decomposed by the action of the catalystand the heat applied in the process of manufacturing the electrodecatalyst layer or more specifically in its drying process to generatesulfate ion (SO₄ ²⁻) and thereby cause poisoning of the electrodecatalyst layer. The inventors have also found that direct combustion ofthe solvent included in the catalyst ink by the action of the catalystand the heat in the drying process accelerates decomposition of theionomer. This causes poisoning of an electrode catalyst layer even in aninitial stage of a fuel cell (or more specifically membrane electrodeassembly) and causes problems such as reduction of the protonconductivity of the electrode catalyst layer and increase in impedanceof the electrode comprised of the electrode catalyst layer and the gasdiffusion layer.

Neither JP 2005-235556A nor JP 2012-212661A describes that combustion ofthe solvent in the catalyst ink accelerates generation of sulfate ion bydecomposition of the ionomer, so as to cause poisoning of the electrodecatalyst layer even in the initial stage of the fuel cell (membraneelectrode assembly) and cause problems such as reduction of the protonconductivity of the electrode catalyst layer and increase in impedanceof the electrode.

SUMMARY

In order to solve at least part of the above problems, the invention maybe implemented by any of the following aspects.

(1) According to one aspect of the invention, there is provided a methodof manufacturing a membrane electrode assembly that has an electrodecatalyst layer formed on a surface of an electrolyte membrane. Thismanufacturing method produces the electrode catalyst layer formed in themembrane electrode assembly by a drying process that dries a catalystink which includes catalyst-supported particles having a catalyst metalsupported thereon, a solvent and an ionomer, at a predeterminedtemperature. The catalyst ink includes a plurality of different solventshaving different boiling points. The predetermined temperature is set tobe lower than the boiling point of the solvent having the lowest boilingpoint among the plurality of different solvents.

The method of manufacturing the membrane electrode assembly according tothis aspect suppresses combustion of the respective solvents included inthe catalyst ink at the drying process and thereby suppressesaccelerative generation of sulfate ion by decomposition of the ionomer.This accordingly suppresses the electrode catalyst layer from beingpoisoned in a resulting fuel cell using this membrane electrode assemblyand remedies problems such as reduction of the proton conductivity ofthe electrode catalyst layer and increase in impedance of the electrodeof the membrane electrode assembly.

(2) In the method of manufacturing the membrane electrode assemblyaccording to the above aspect, the catalyst ink may include at least twodifferent solvents having different boiling point. The drying processmay comprise a first drying process that sets the predeterminedtemperature to a temperature that is lower than the boiling point of thesolvent having the lower boiling point between the two differentsolvents; and a second drying process that is performed after the firstdrying process and sets the predetermined temperature to be higher thanthe temperature in the first drying process but lower than the boilingtemperature of the other solvent.

The solvent having the lower boiling point is more easily evaporated anddried, After completion of evaporation of a solvent, even a temperaturerise to be higher than the boiling point of the solvent does not causesulfate ion to be generated by combustion of the solvent. The firstdrying process dries one solvent having the lower boiling point at thetemperature lower than the boiling point of the one solvent. The seconddrying process dries the other solvent at the temperature that is higherthan the temperature in the first drying process but is lower than theboiling point of the other solvent. This suppresses generation ofsulfate ion by combustion of the respective solvents, while reducing thetime duration required for drying.

(3) The method of manufacturing the membrane electrode assemblyaccording to the above aspect may further comprise, after the dryingprocess, measuring an amount of sulfate ion included in the producedelectrode catalyst layer and evaluating an electrode catalyst layerhaving the measured amount of sulfate ion that is equal to or less thana specified reference value, as a good product.

The method of manufacturing the membrane electrode assembly according tothis aspect readily selects the electrode catalyst layer that has asmall amount of sulfate ion and has a low potential for poisoning, basedon the specified reference value, to be used as a non-defectiveelectrode catalyst layer. This accordingly suppresses the electrodecatalyst layer from being poisoned in a resulting fuel cell using themanufactured membrane electrode assembly and remedies problems such asreduction of the proton conductivity of the electrode catalyst layer andincrease in impedance of the electrode of the membrane electrodeassembly.

(4) In the method of manufacturing the membrane electrode assemblyaccording to the above aspect, the reference value may be an amount ofsulfate ion corresponding to an inflection point of output currentdensity obtained from a relationship that is specified in advancebetween amount of sulfate ion included in the electrode catalyst layerin unused state and output current density of a fuel cell using theelectrode catalyst layer.

In the method of manufacturing the membrane electrode assembly accordingto this aspect, the reference value is set to a value that suppressespoisoning of the electrode catalyst layer and remedies problems such asreduction of the proton conductivity of the electrode catalyst layer andincrease in impedance of the electrode of the membrane electrodeassembly. This facilitates selection of the electrode catalyst layer tobe used as a non-defective electrode catalyst layer for the membraneelectrode assembly.

(5) In the method of manufacturing the membrane electrode assemblyaccording to the above aspect, the reference value may be 0.33 μg/cm².

(6) According to another aspect of the invention, there is provided amembrane electrode assembly that has an electrode catalyst layer formedon a surface of an electrolyte membrane. The electrode catalyst layerincludes an ionomer and catalyst-supported particles having a catalystmetal supported thereon. An amount of sulfate ion included in theelectrode catalyst layer in unused state is equal to or less than aspecified reference value.

In the membrane electrode assembly according to this aspect, the amountof sulfate ion included in the electrode catalyst layer is equal to orless than the specified reference value. This suppresses the electrodecatalyst layer from being poisoned in a resulting fuel cell using themembrane electrode assembly including this electrode catalyst layer andremedies problems such as reduction of the proton conductivity of theelectrode catalyst layer, increase in impedance of the electrode of themembrane electrode assembly and reduction of power generationperformance of the fuel cell.

(7) in the membrane electrode assembly according to the above aspect,the reference value may be an amount of sulfate ion corresponding to aninflection point of output current density obtained from a relationshipthat is specified in advance between amount of sulfate ion included inthe electrode catalyst layer in unused state and output current densityof a fuel cell using the electrode catalyst layer.

In the membrane electrode assembly of this aspect, the reference valueis set to a value that suppresses poisoning of the electrode catalystlayer and remedies problems such as reduction of the proton conductivityof the electrode catalyst layer, increase in impedance of the electrodeof the membrane electrode assembly and reduction of power generationperformance of the fuel cell. This suppresses the electrode catalystlayer from being poisoned in a resulting fuel cell using the membraneelectrode assembly including this electrode catalyst layer and remediesproblems such as reduction of the proton conductivity of the electrodecatalyst layer and increase in impedance of the electrode of themembrane electrode assembly.

(8) in the membrane electrode assembly according to the above aspect,the reference value may be 0.33 μg/cm².

The invention may be implemented by any of various aspects other thanthe membrane electrode assembly and the method of manufacturing themembrane electrode assembly described above, for example, an electrodecatalyst layer, a fuel cell, a method of manufacturing an electrodecatalyst layer and a method of manufacturing a fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing a method of manufacturing a membraneelectrode assembly used for a fuel cell according to one embodiment;

FIG. 2 is a flowchart showing a production process of an electrodecatalyst layer;

FIG. 3 is a diagram illustrating an example of a process of coating asheet with catalyst ink and a process of drying a coated layer ofcatalyst ink;

FIG. 4 is a diagram showing one example of drying temperature history ina drying furnace;

FIG. 5 is a diagram showing another example of drying temperaturehistory in the drying furnace;

FIG. 6 is a graph showing a relationship between drying time durationand amount of sulfate ion in the electrode catalyst layer;

FIG. 7 is a flowchart showing an inspection process of the electrodecatalyst layer;

FIG. 8 is a flowchart showing a procedure of measuring the amount ofsulfate ion in the electrode catalyst layer;

FIG. 9 is a diagram illustrating one example of an analyzer foranalyzing the ion component by ion chromatography;

FIG. 10 is a graph showing a relationship between amount, of sulfate ionincluded in an electrode catalyst layer and output current density of afuel cell using the electrode catalyst layer;

FIG. 11 is a diagram illustrating a catalyst coated membrane produced byusing an electrolyte membrane and electrode catalyst layers;

FIG. 12 is a diagram illustrating a membrane electrode assembly producedby using the catalyst coated membrane and gas diffusion layers; and

FIG. 13 is a diagram illustrating a fuel cell configured by using themembrane electrode assembly.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a flowchart showing a method of manufacturing a membraneelectrode assembly used for a fuel cell according to one embodiment.This manufacturing method provides an electrolyte membrane (step S100),provides electrode catalyst layers (step S200), produces a catalystcoated membrane (CCM) by using the provided electrolyte membrane andelectrode catalyst layers (step S300), provides gas diffusion layers(GEL) (step S400) and produces a membrane electrode assembly (MEM byusing the produced catalyst coated membrane and the provided gasdiffusion layers (step S500), as described in detail below.

The electrolyte membrane provided at step S100 is a proton-conductiveion exchange resin membrane that is made of an ionomer having a sulfonicacid group as an end group, like an ionomer included in an electrodecatalyst layer described later. This embodiment uses a Nafion membranemade of Nafion (registered trademark) as the electrolyte membrane.

At step S200, electrode catalyst layers are provided by producingelectrode catalyst layers and examining the produced electrode catalystlayers as described below.

FIG. 2 is a flowchart showing a production process of the electrodecatalyst layer. At step S210, catalyst ink is provided. The catalyst inkmay be produced (prepared) by for example, the following process.Catalyst-supported particles provided are mixed with water (ion exchangewater) and are subsequently mixed with a plurality of hydrophilicsolvents (hereinafter simply called “solvents”) such as ethanol andpropanol and an ionomer provided. The resulting mixture is dispersedusing, for example, an ultrasonic homogenizer or a bead mill, so thatthe catalyst ink is produced. The foregoing process is, however, notrestrictive, and the catalyst ink may be produced by any of variousconventional techniques.

The catalyst-supported particles may be produced by, for example, thefollowing process. Conductive particles for supporting that are capableof supporting a catalyst metal are dispersed in a solution of thecatalyst metal, and the catalyst-supported particles are produced byimpregnation method, coprecipitation method, ion exchange method or thelike. The particles for supporting may be selectable from various carbonparticles (carbon powders). For example, carbon black or carbonnanotubes may be used as the particles for supporting. The catalystmetal used may be platinum or a platinum compound (for example,platinum-cobalt alloy or platinum-nickel alloy). The ionomer used forproduction of the electrode catalyst layer is a proton-conductiveelectrolyte material having a sulfonic acid group as an end group. Thisembodiment employs Nafion (registered trademark) for the ionomer, likethe electrolyte membrane. The foregoing process is, however, notrestrictive, and the catalyst-supported particles may be produced by anyof various conventional technique.

The production process subsequently coats a sheet (also called “basedmaterial”) with the catalyst ink to form a coated layer of catalyst inkat step S220 and dries the coated layer of catalyst ink to form anelectrode catalyst layer on the sheet at step S230.

FIG. 3 is a diagram illustrating an example of the process of coating asheet with the catalyst ink and the process of drying a coated layer ofcatalyst ink. As shown in FIG. 3, at step S220, a long sheet BS woundoff from a sheet roll BSr is coated with the catalyst ink by using acoater (for example, die coater) 50, so that a coated layer of catalystink Licat is formed on the sheet BS.

Referring back to 2, at step S230, the coated layer of catalyst inkLicat formed on the sheet BS is dried by a drying process (heatingprocess) in a drying furnace 60, so that an electrode catalyst layer Lctis formed on the sheet BS. The sheet BS with the electrode catalystlayer Lct formed on the surface thereof is wound on a roll as anelectrode catalyst layer sheet roll Csr.

FIG. 4 is a diagram showing one example of drying temperature history inthe drying furnace. The coated layer of catalyst ink Licat fed into thedrying furnace 60 of FIG. 3 is sequentially heated to dryingtemperatures according to a drying temperature history set in the dryingfurnace 60 and is dried. In the example of drying temperature historyshown in FIG. 4, the catalyst ink includes three different solvents S1,S2 and S3 and water. The first solvent S1 is acetone (boiling point Tb1:56.5° C.), the second solvent S2 is ethanol (boiling point Tb2: 78.5°C.), and the third solvent S3 is 1-propanol (boiling point Tb3: 97.2°C.).

The drying process sequentially dries the coated layer of catalyst inkLicat in the drying furnace 60. The coated layer of catalyst ink Licatis first dried at a first temperature (also called “first dryingtemperature”) Ts1 that is lower than the first boiling point Tb1 of thefirst solvent S1 for a first time duration (also called “first dryingtime duration”) ps1. The coated layer of catalyst ink Licat issubsequently dried at a second drying temperature Ts2 that is higherthan the first boiling temperature Tb1 but is lower than the secondboiling temperature Tb2 of the second solvent S2 for a second dryingtime duration ps2. The coated layer of catalyst ink Licat is then driedat a third drying temperature Ts3 that is higher than the second boilingtemperature Tb2 but is lower than the third boiling temperature Tb3 ofthe third solvent S3 for a third drying time duration ps3. As a result,the three different solvents S1, S2 and S3 are respectively evaporated.In the course of evaporation of these three different solvents S1, S2and S3, water included in the catalyst ink is also evaporated. Thisalmost completely dries the coated layer of catalyst ink Licat (by 95%or higher in this example). The coated layer of catalyst ink Licat islastly heated to a heating temperature Ts4 (for example, 140° C.) thatis higher than the third boiling point Tb3 and boiling point Tb4 ofwater (100° C.) but is not higher than an upper limit temperature (forexample, 150° C.) for a heating time ps4. This series of drying andheating process thus sequentially dries the coated layer Licat ofcatalyst ink fed into the drying furnace 60 to form the electrodecatalyst layer Lct.

As described above, the drying and heating process with the dryingfurnace 60 is performed according to the drying temperature history(shown in FIG. 4) at step S230. The drying process is performed tosequentially dry the coated layer Licat of catalyst ink at the dryingtemperatures Ts1, Ts2 and Ts3, which respectively depend on the boilingpoints Tb1, Tb2 and Tb3 of the three different solvents S1, S2 and S3included in the catalyst ink. The heating process is subsequentlyperformed to heat the coated layer Licat of catalyst ink at the heatingtemperature Ts4 that is higher than the highest third boiling point Tb3and the boiling point Tb4 of water, so as to form the electrode catalystlayer Lct on the sheet BS.

The three-stage drying temperatures Ts1, Ts2 and Ts3 are respectivelyset to be lower than the boiling points Tb1, Tb2 and Tb3 of therespective solvents S1, S2 and S3 as described above. By taking intoaccount some margins, it is preferable that the drying temperatures Ts1,Ts2 and Ts3 are respectively lower than the boiling points Tb1, Tb2 andTb3 of the respective solvents S1, S2 and S3 by at least 5° C. Settingthe drying temperature to be significantly lower than the boiling point,however, increases the drying time duration. By taking into account thetime efficiency, it is preferable that the drying temperature is asclose as the boiling temperature. In this example, the respective dryingtemperatures Ts1, Ts2 and Ts3 are set to be lower than the boilingpoints Tb1, Tb2 and Tb3 of the respective solvents S1, S2 and S3 by 5°C. as follows:Ts1=[Tb1·5]=51.5° C.;Ts2=[Tb2·5]=73.5° C.;Ts3=[Tb3·5]=92.2° C.;

The drying time durations ps1, ps2 and ps3 at the respective dryingtemperatures Ts1, Ts2 and Ts3 are set to time durations required forevaporation and drying, according to the amounts of the respectivesolvents. The drying time durations ps1, ps2 and ps3 and the heatingtime ps4 are sequentially set from the inlet side toward the outlet sidein the drying furnace 60 and are determined according to the lengths ofrespective sections set at the respective drying temperatures Ts1, Ts2,Ts3 and Ts4 in the drying furnace 60 and the feeding speed.

The heating to Ts4 is not specifically limited, as long as the heatingtemperature Ts4 is higher than the boiling point of the solvent havingthe highest boiling point (third boiling point Tb3 of the third solvent53 in this example) and the boiling point Tb4 of water and is not higherthan the upper limit temperature. The upper limit temperature ispreferably not higher than 150° C. and is more preferably not higherthan 145° C. The heating process at the heating temperature Ts4 may beomitted as appropriate. In the case where this heating process isomitted, however, it is preferable to set the drying time durations atthe respective drying temperatures to complete drying by the dryingprocess at the three-stage drying temperatures.

The solvent having the lower boiling point is more easily evaporated anddried. After completion of evaporation of a solvent, even a temperaturerise to be higher than the boiling point of the solvent does not causesulfate ion to be generated by combustion of the solvent. Accordingly,the drying process at the gradually increased temperatures depending onthe points of the respective solvents like the drying temperaturehistory described above suppresses generation of sulfate ion bycombustion of the solvent and reduces the time duration required fordrying.

FIG. 5 is a diagram showing another example drying temperature historyin the drying furnace. The conditions of the catalyst ink are identicalwith those in FIG. 4. The drying temperature history of FIG. 4 performsthe drying process at the gradually increased three-stage temperaturesTs1, Ts2 and Ts3. The drying temperature history of FIG. 5, however,almost completes drying (95% or more) at the first drying temperatureTs1 with respect to the first solvent S1 having the lowest boiling point(acetone in the illustrated example) and subsequently performs theheating process at the heating temperature Ts4 for a heating time ps4m.

The drying process according to the drying temperature history of FIG. 5dries the coated layer of catalyst ink Licat at the drying temperaturedetermined depending on the boiling point of the solvent having thelowest boiling point, thus suppressing generation of sulfate ion bycombustion of the solvent. The drying temperature history of FIG. 5requires the longer time duration for drying, compared with the dryingtemperature history of FIG. 4.

The heating process may also be omitted in the drying temperaturehistory of FIG. 5. In the case where this heating process is omitted,however, it is preferable to set the drying time duration to completedrying at the drying temperature determined depending on the boilingpoint of the solvent having the lowest boiling point.

In the above examples, the catalyst ink includes three differentsolvents S1, S2 and S3 and water. In another example, the catalyst inkmay include only one type of solvent and water. In this example, thecoated layer of catalyst ink may be dried at a drying temperature thatis lower than the boiling point of this one single solvent. In anotherexample, the catalyst ink may include four or more different solvents.In this example, the coated layer of catalyst ink may be driedsequentially at drying temperatures that are increased gradually and arerespectively set to be lower than the boiling points of thecorresponding single solvents in the sequence from the solvent havingthe lowest boiling point to the solvent having the highest boilingpoint.

FIG. 6 is a graph showing a relationship between the drying timeduration and the amount of sulfate ion in the electrode catalyst layer,FIG. 6 shows the results of measurement of sulfate ion in electrodecatalyst layers produced at a fixed drying temperature of 150° C. fordrying time durations of 60 minutes, 80 minutes, 100 minutes and 140minutes. The conditions of the catalyst ink are identical with those inFIGS. 4 and 5. The amount of sulfate ion is measured by analysis of ioncomponents included in an extract that is obtained by soaking theelectrode catalyst layer in warm water, by ion chromatography.

As indicated by FIG. 6, even at the drying temperature of 150° C. thatis higher than the boiling points of the solvents S1, S2 and S3 settingthe drying time duration to be not longer than 80 minutes suppressesgeneration of sulfate ion. Setting the short drying time duration at thehigher drying temperature than the boiling point of the solvent cansuppress generation of sulfate ion by combustion of the solvent. Thedrying temperature and the drying time duration may be determinedaccording to the catalyst ink used by experimentally checking in advancethe temperature and the time duration that evaporate all the solvents tocomplete drying and suppress generation of sulfate ion by combustion ofthe solvent.

FIG. 7 is a flowchart showing an inspection process of the electrodecatalyst layer. The inspection process measures the amount of sulfateion included in the electrode catalyst layer (step S240) and determineswhether the amount of sulfate ion is equal to or less than a specifiedreference value Ar [μg/cm²] (step S250), A concrete example of thereference value Ar will be described later.

FIG. 8 is a flowchart showing a procedure of measuring the amount ofsulfate ion in the electrode catalyst layer. The procedure firsttransfers part of the electrode catalyst layer Lct of the electrodecatalyst layer sheet roll Csr (shown in FIG. 3) onto a transfer sheetTCS to obtain a test piece (step S242). Any of various resin sheets suchas polyimide sheet may be used as the transfer sheet TCS. This exampleuses a Kapton (registered trademark) sheet. The procedure stacks thetransfer sheet TCS on a surface of the electrode catalyst layer Lct,presses the stacked layers and treats the pressed layers by heattreatment at a heating temperature of 150° C. for a heating timeduration of 1 hour, so as to transfer the electrode catalyst layer Letonto the transfer sheet TCS. A test piece may be obtained by cutting thetransferred electrode catalyst sheet Lct into, for example, a size of 39cm². The size of the test piece is not specifically limited to thissize.

The procedure subsequently soaks the obtained test piece in warm waterto obtain an extract (step S244). In this example, an extract isobtained by soaking the test piece in 45 mL of pure water at atemperature of 90° C. for an extraction time duration of 20 hours. Theconditions of warm water immersion are not limited to these conditionsbut may be any suitable conditions that enable sulfate ion as themeasuring object to be sufficiently extracted from the electrodecatalyst layer of the test piece.

The procedure then analyzes the ion component included in the extract byion chromatography to measure the amount of sulfate ion (step S246).FIG. 9 is a diagram illustrating one example of an analyzer foranalyzing the ion component by ion chromatography (also called “ionchromatograph”). This analyzer 100 includes an eluent pump 170, a sampleloop 160, a guard column 150, a separation column 140, a suppressor 130,an electrical conductivity detector 120 and an analysis computer 110 byion chromatography.

The extract as the sample is injected into the sample loop 160, istransmitted with the eluent, which is fed by the eluent pump 170,through the guard column 150 to the separation column 140 and isseparated by the strength of interaction with a filler in the separationcolumn 140 (mainly ion exchange action). The suppressor 130 suppressesthe electrical conductivity of the eluent, in order to prevent theelectrical conductivity of the eluent from affecting the electricalconductivity of the ion component in the extract. The ion component ofthe extract separated by the separation column 140 is measured by theelectrical conductivity detector 120. The measurement result by theelectrical conductivity detector 120 is analyzed by the analysiscomputer 110 to give a chromatogram. The amount of sulfate ion in thetest piece is determined from this chromatogram. The amount of sulfateion included in the entire electrode catalyst layer may be determined byestimation of the value corresponding to the size of the electrodecatalyst layer from the amount of sulfate ion in the test piece.

FIG. 10 is a graph showing one example of relationship between theamount of sulfate ion included in an electrode catalyst layer and theoutput current density of a fuel cell using the electrode catalystlayer. The amount of sulfate ion is an amount per unit area of theelectrode catalyst layer (electrode catalyst layer having the thicknessof 10 measured by the procedure of measuring the amount of sulfate iondescribed above. The amount of sulfate ion may be increased with anincrease in drying temperature of the catalyst ink and decreased with adecrease in drying temperature as described above. The amount of sulfateion may also be increased with an increase in drying time duration ofthe catalyst ink and decreased with a decrease in drying time duration.

As shown in FIG. 10, the output current density decreases with anincrease in amount of sulfate ion. More specifically, the output currentdensity has a higher decrease rate when the amount of sulfate ion islarger than a certain amount of sulfate ion Ar (0.33 μg/cm² in thisexample) as the boundary (inflection point). The output current densityhas a lower decrease rate when the amount of sulfate ion is equal to orless than the certain amount of sulfate ion Ar. Accordingly controllingthe amount of sulfate ion in the electrode catalyst layer to be equal toor less than the amount of sulfate ion Ar as the inflection pointsuppresses the decrease of the output current density and therebyreduction in output of a resulting fuel cell. This amount of sulfate ionAr is set to the reference value Ar, and the inspection process of FIG.7 determines whether the measured amount of sulfate ion is equal to orless than the reference value Ar at step S250 as described above.

When the measured amount of sulfate ion is larger than the referencevalue Ar, the electrode catalyst layer is judged to provide poor outputand evaluated as unusable defective product (NG product) (step S260 b).When the measured amount of sulfate ion is equal to or less than thereference value Ar, on the other hand, the electrode catalyst layer isjudged to provide good output and evaluated as usable good product (OKproduct) (step S260 a). The electrode catalyst layer sheet roll Carevaluated as OK product is used for production of a catalyst coatedmembrane (CCM) described below.

The amount of sulfate ion Ar as the inflection point (reference valueAr) differs according to the conditions of preparing the catalyst ink(for example, catalyst-supported particles, solvent and the compositionof ionomer) and the conditions of soaking the test sample in warm waterand may thus be determined experimentally according to the conditions ofthe catalyst ink used and warm water immersion. The reference value Ar(=0.33 μg/cm²) in FIG. 10 is only illustrative, and it is generallypreferable to set the reference value Ar in the range of 0.2 to 0.35.

In the inspection of the electrode catalyst layer described above, theamount of sulfate ion in the electrode catalyst layer is measured bytransferring the electrode catalyst layer onto the transfer sheet TCS.This is for the purpose of causing a surface of the electrode catalystlayer that is in contact with a gas diffusion layer in production of amembrane electrode assembly to be directly exposed to warm water in warmwater immersion and thereby enhancing the extraction accuracy of sulfateion on the surface. As described previously, a radical scavenger (forexample, cerium oxide) that is eluted in the presence of sulfate ion andleads to poisoning of the electrode catalyst layer is included in thegas diffusion layer. The sulfate ion on the surface of the electrodecatalyst layer that is in contact with the gas diffusion layer is thusexpected to have significant effect. The heating process in the processof transfer of the electrode catalyst layer onto the transfer sheet TCSis expected to enhance the extraction accuracy of sulfate ion includedin the transferred electrode catalyst layer. As shown in FIG. 10,measurement of the amount of sulfate ion using an extract of a testpiece obtained by cutting the electrode catalyst layer sheet roll Csr(measurement without transfer) has the lower measurement accuracy of theamount of sulfate ion, compared with measurement with transfer. Thisleads to a failure in specifying the amount of sulfate ion Ar as theinflection point with high accuracy. A test piece obtained by theheating process without transferring the electrode catalyst layer ontothe transfer sheet TCS allows for measurement of the amount of sulfateion with high accuracy. Transferring the electrode catalyst sheet ontothe transfer sheet TCS however, preferable, since the surface of theelectrode catalyst layer that is in contact with the gas diffusion layeris directly exposed to warm water for extraction.

As described above, at step S200 in FIG. 1, the electrode catalyst layerused for production of a catalyst coated membrane described belowprovided by producing the electrode catalyst layer (as shown in FIGS. 2to 4) and examining the produced electrode catalyst layer (as shown inFIGS. 7 to 10).

FIG. 11 is a diagram illustrating a catalyst coated membrane produced byusing the electrolyte membrane and the electrode catalyst layers. Atstep S300 in FIG. 1, the electrode catalyst layers 23 and 24 provided atstep S200 are placed on the respective surfaces of the electrolytemembrane 22 provided at step S100 and are hot pressed. This provides acatalyst coated membrane 21 that has the electrode catalyst layer 23formed on (joined with) one surface of the electrolyte membrane 22 andthe electrode catalyst layer 24 formed on the other surface of theelectrolyte membrane 22.

At step S400 in FIG. 1, gas diffusion layers used for production of amembrane electrode assembly are provided. The gas diffusion layers aremade of a gas-permeable conductive material, for example, carbon porousmaterial such as carbon cloth or carbon paper or a metal porous materialsuch as metal mesh or metal foam. The gas diffusion layers areimpregnated with a radical scavenger (for example, cerium oxide).

FIG. 12 is a diagram illustrating a membrane electrode assembly producedby using the catalyst coated membrane and the gas diffusion layers. Atstep S500 in FIG. 1, the gas diffusion layers 25 and 26 provided at stepS400 are placed on the respective surfaces of the catalyst coatedmembrane 21 produced at step S300 and are hot pressed. This provides amembrane electrode assembly 20 that has the gas diffusion layer 25formed on (joined with) a surface of the electrode catalyst layer 23 ofthe catalyst coated membrane 21 and the gas diffusion layer 26 formed ona surface of the electrode catalyst layer 24 of the catalyst coatedmembrane 21. The catalyst coated membrane 21 may be called “membraneelectrode assembly”, and the membrane electrode assembly 20 may becalled “membrane electrode and gas diffusion layer assembly (MEGA).

For the simple explanation, FIG. 11 illustrates producing the catalystcoated membrane from the electrode catalyst layers and the electrolytemembrane in the sheet form, and FIG. 12 illustrates producing themembrane electrode assembly from the catalyst coated membrane and thegas diffusion layers in the sheet form. The invention is, however, notlimited to this configuration. Long electrode catalyst layers may be hotpressed on a long electrolyte membrane, or a plurality of electrodecatalyst layers in the sheet form may be hot pressed on a longelectrolyte membrane at predetermined intervals. Additionally, aplurality of gas diffusion layers in the sheet form may be further hotpressed at predetermined intervals. This produces a continuous sheet ofa plurality of membrane electrode assemblies, which may be subsequentlycut into individual pieces.

FIG. 13 is a diagram illustrating a fuel cell configured by using themembrane electrode assembly. A fuel cell 10 is configured by placing themembrane electrode assembly 20 shown in FIG. 12 between a separator 27located on the anode (electrode catalyst layer 23 and gas diffusionlayer 25) side and a separator 28 located on the cathode (electrodecatalyst layer 24 and gas diffusion layer 26) side.

The separators 27 and 28 are made of a gas-impermeable conductivematerial, for example, dense carbon obtained by compressing carbon to begas impermeable or press-molded metal plate. Surfaces of the separators27 and 28 placed to be in contact with the membrane electrode assembly20 have concavity and convexity to form flow paths for a fuel gas and anoxidizing gas. More specifically, fuel gas flow paths 27 p for the flowof fuel gas (H₂) subjected to the electrochemical reaction at the anodeare formed between the gas diffusion layer 25 and the separator 27 onthe anode side. Oxidizing gas flow paths 28 p for the flow of oxidizinggas (O₂ or more specifically the air including O₂) subjected to theelectrochemical reaction at the cathode are formed between the gasdiffusion layer 26 and the separator 28 on the cathode side.

In the actual use, fuel cells are generally used in the form of a fuelcell stack having the stacked structure of a plurality of the fuel cells10 shown in FIG. 13.

In the embodiment described above, the drying temperature is set to belower than the boiling point of the solvent in the process ofmanufacturing the electrode catalyst layer or more specifically in theprocess of drying the coated layer of catalyst ink, so as to suppressgeneration of sulfate ion by combustion of the solvent. This allows forproduction of the electrode catalyst layer that has the reduced amountof sulfate ion generated in the process of manufacturing the electrodecatalyst layer.

The amount of sulfate ion in the produced electrode catalyst layer ismeasured, and the electrode catalyst layer having the amount of sulfateion that is equal to or less than the specified reference value is usedfor production of the membrane electrode assembly. The reference valueis an amount of sulfate ion (for example, 0.33 μg/cm²) at an inflectionpoint of output current density obtained from a relationship that isspecified in advance between the amount of sulfate ion included in theelectrode catalyst layer in unused state and the output current densityof a fuel cell using the electrode catalyst layer (as shown in FIG. 10).This suppresses the electrode catalyst layer from being poisoned bysulfate ion included in the electrode catalyst layer in the initialstage in the fuel cell using the membrane electrode assembly. Thisaccordingly suppresses reduction of the proton conductivity of theelectrode catalyst layer and increase in impedance of the electrode ofthe membrane electrode assembly, thus suppressing reduction of the powergeneration performance of the fuel cell.

In the catalyst coated membrane 21 shown in FIG. 11 and in the membraneelectrode assembly 20 shown in FIG. 12, both the electrode catalystlayers 23 and 24 on the respective sides of the electrolyte membrane 22have the amounts of sulfate ion equal to or less than the referencevalue Ar (0.33 μg/cm² in the above example). According to amodification, only either one of the electrode catalyst layers 23 and 24may have the amount of sulfate ion equal to or less than the referencevalue Ar.

In the embodiment described above, the electrode catalyst layers 23 and24 are produced by coating the sheet BS with the catalyst ink and dryingthe catalyst coated sheet (as shown in step S220 in FIG. 2 and FIG. 3).One modification may produce the electrode catalyst layer without usingthe sheet BS by directly coating the electrolyte membrane 22 with thecatalyst ink and drying the catalyst coated electrolyte membrane 22.This modification forms electrode catalyst layers 23 and 24 by coatingthe electrolyte membrane 22 with the catalyst ink and drying thecatalyst coated electrolyte membrane 22 so as to form the catalystcoated membrane 21, while the embodiment joins the electrode catalystlayers 23 and 24 with the electrolyte membrane 22 by hot pressing so asto form the catalyst coated membrane 21 (shown in step S300 in FIG. 1and FIG. 11).

In the fuel cell 10 shown in FIG. 13, the channel-like gas flow paths 27p and 28 p are formed in the separators 27 and 28 which are arrangedacross the membrane electrode assembly 20. This configuration is,however, not restrictive. Gas flow paths, for example, porous gas flowpaths, may be provided separately between the separators and themembrane electrode assembly. Such gas flow paths may be providedseparately between either one of the separators and the membraneelectrode assembly.

The invention is not limited to any of the embodiments, the examples andthe modifications described above but may be implemented by a diversityof other configurations without departing from the scope of theinvention. For example, the technical features of any of theembodiments, examples and modifications corresponding to the technicalfeatures of each of the aspects described in Summary may be replaced orcombined appropriately, in order to solve part or all of the problemsdescribed above or in order to achieve part or all of the advantageouseffects described above. Any of the technical features may be omittedappropriately unless the technical feature is described as essentialherein.

What is claimed is:
 1. A method of manufacturing a membrane electrodeassembly that has an electrode catalyst layer formed on a surface of anelectrolyte membrane, the method comprising: producing the electrodecatalyst layer formed in the membrane electrode assembly by a dryingprocess that dries a catalyst ink which includes catalyst-supportedparticles having a catalyst metal supported thereon, a solvent, and anionomer, at a predetermined temperature; and after the drying process,measuring an amount of sulfate ion included in the produced electrodecatalyst layer and evaluating an electrode catalyst layer having themeasured amount of sulfate ion that is equal to or less than a specifiedreference value, as a good product, wherein the reference value is 0.33pg/cm2, the catalyst ink includes a plurality of different solventshaving different boiling points, the predetermined temperature is set tobe lower than the boiling point of the solvent having the lowest boilingpoint among the plurality of different solvents, and the catalyst inkincludes a first solvent having a first boiling point and a secondsolvent having a second boiling point higher than the first boilingpoint, and wherein the drying process comprises: a first heating processthat sets the predetermined temperature to a temperature that is higherthan the first boiling point and lower than the second boiling point fora first time duration, the first solvent being completely evaporated bythe end of the first time duration; a second heating process that isperformed after the first heating process and sets the predeterminedtemperature to be higher than the predetermined temperature in the firstheating process and higher than the second boiling point for a secondtime duration, the second solvent being completely evaporated by the endof the second time duration; a third heating process that is performedbefore the first heating process and sets the predetermined temperatureto a temperature that is lower than the first boiling point; and afourth heating process that is performed after the second heatingprocess and sets the predetermined temperature to be higher than thetemperature in the second heating process.
 2. The method ofmanufacturing the membrane electrode assembly according to claim 1,wherein the reference value is an amount of sulfate ion corresponding toan inflection point of output current density obtained from arelationship that is specified in advance between amount of sulfate ionincluded in the electrode catalyst layer in unused state and outputcurrent density of a fuel cell using the electrode catalyst layer.